A-John lost his job last month and since then he has been out of work.
-Do you know why he lost his job?
-Because he was very rude to his boss.
Fresh air is of great use to our health.
They are the children who won the match yesterday.

Explanation:
1. "A-John lost his job last month and since then he has been out of work." - The verb "lose" is used in the past tense to indicate that John lost his job in the past. The verb "be" is used in the present perfect tense to indicate that he has been out of work since then.
2. "Do you know why he lost his job?" - The verb "lose" is used in the past tense to ask about the reason for John losing his job.
3. "Because he was very rude to his boss." - The verb "be" is used in the past tense to describe John's behavior of being rude to his boss, which resulted in him losing his job.
4. "Fresh air is of great use to our health." - The preposition "of" is used to indicate the relationship between fresh air and its use for our health.
5. "They are the children who won the match yesterday." - The relative pronoun "who" is used to refer to the children who won the match yesterday.